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We present an abundance analysis of 101 subgiant branch (SGB) stars in theglobular cluster M22. Using low resolution FLAMES/GIRAFFE spectra we havedetermined abundances of the neutron-capture strontium and barium and the lightelement carbon. With these data we explore relationships between the observedSGB photometric split in this cluster and two stellar groups characterized bydifferent contents of iron, slow neutron-capture process (s-process) elements,and the alpha element calcium, that we previously discovered in M22's red-giantstars. We show that the SGB stars correlate in chemical composition andcolor-magnitude diagram position: the stars with higher metallicity andrelative s-process abundances define a fainter SGB, while stars with lowermetallicity and s-process content reside on a relatively brighter SGB. Thisresult has implications for the relative ages of the two stellar groups of M22.In particular, it is inconsistent with a large spread in ages of the two SGBs.By accounting for the chemical content of the two stellar groups, isochronefitting of the double SGB suggests that their ages are not different by morethan 300 Myr.

The double sub-giant branch of NGC 6656 (M22): a chemical characterization